There are optical coupling devices, including photorelays and photocouplers, having “double mold” structures. Generally, in a “double mold” structure, a light-emitting element and a light-receiving element are first covered with an inner mold resin, which is then covered with an outer mold resin.
In these “double mold” structures, stress is applied to the elements within when the device's ambient temperature changes, and thus there is a concern about device deteriorating due to this applied stress, particularly with regard to the light-emitting element within the device. A method has been adopted in which a light-emitting element is first covered with a silicone resin, which is then covered with the inner mold resin. The silicone resin is considered to provide a means of relaxing applied or induced stress which might otherwise affect the light-emitting element.
However, the temperature characteristics of the silicone resin are typically considerably different from the temperature characteristics of the inner mold resins in common usage. For this reason, when temperature changes occur, there will be a tendency for separation (peeling) to occur at the interface between the silicone resin and the inner mold resin due to, for example, differences in thermal expansion coefficients. When peeling occurs at this interface, a light path between the light-emitting element and the light-receiving element is degraded or blocked. Thereby, the amount of light received by the light-receiving element will be decreased, and thus there is a concern of light utilization efficiency being reduced.